diff --git a/llama-index-core/llama_index/core/base/response/schema.py b/llama-index-core/llama_index/core/base/response/schema.py
index f3239338bbab2c0f8d9134c96c83872eff89b1a4..3689ed2cefa514d14f8da2357c2965cf0e4a7404 100644
--- a/llama-index-core/llama_index/core/base/response/schema.py
+++ b/llama-index-core/llama_index/core/base/response/schema.py
@@ -159,6 +159,16 @@ class AsyncStreamingResponse:
     def __post_init__(self) -> None:
         self._lock = asyncio.Lock()
 
+    def __str__(self) -> str:
+        """Convert to string representation."""
+        return asyncio.run(self._async_str)
+
+    async def _async_str(self) -> str:
+        """Convert to string representation."""
+        async for _ in self._yield_response():
+            ...
+        return self.response_txt or "None"
+
     async def _yield_response(self) -> TokenAsyncGen:
         """Yield the string response."""
         async with self._lock:
diff --git a/llama-index-core/llama_index/core/instrumentation/events/synthesis.py b/llama-index-core/llama_index/core/instrumentation/events/synthesis.py
index 71251eaec63602d7fb68437261502beaae4139c5..27a3a03dd29baf957b1f10806c28d86894fac8df 100644
--- a/llama-index-core/llama_index/core/instrumentation/events/synthesis.py
+++ b/llama-index-core/llama_index/core/instrumentation/events/synthesis.py
@@ -1,4 +1,3 @@
-from llama_index.core.base.response.schema import RESPONSE_TYPE
 from llama_index.core.instrumentation.events.base import BaseEvent
 from llama_index.core.schema import QueryType
 
@@ -14,7 +13,7 @@ class SynthesizeStartEvent(BaseEvent):
 
 class SynthesizeEndEvent(BaseEvent):
     query: QueryType
-    response: RESPONSE_TYPE
+    response: str
 
     @classmethod
     def class_name(cls):
diff --git a/llama-index-core/llama_index/core/instrumentation/span_handlers/simple.py b/llama-index-core/llama_index/core/instrumentation/span_handlers/simple.py
index 4cdee9d67d88f86bca92cfcd7e979e4fc4816e3c..1cea445b77b880599134547c45df87b5bb87ee7a 100644
--- a/llama-index-core/llama_index/core/instrumentation/span_handlers/simple.py
+++ b/llama-index-core/llama_index/core/instrumentation/span_handlers/simple.py
@@ -3,6 +3,7 @@ from llama_index.core.bridge.pydantic import Field
 from llama_index.core.instrumentation.span.simple import SimpleSpan
 from llama_index.core.instrumentation.span_handlers.base import BaseSpanHandler
 from datetime import datetime
+import warnings
 
 if TYPE_CHECKING:
     from treelib import Tree
@@ -42,6 +43,7 @@ class SimpleSpanHandler(BaseSpanHandler[SimpleSpan]):
         """Method for getting trace trees."""
         try:
             from treelib import Tree
+            from treelib.exceptions import NodeIDAbsentError
         except ImportError as e:
             raise ImportError(
                 "`treelib` package is missing. Please install it by using "
@@ -59,12 +61,31 @@ class SimpleSpanHandler(BaseSpanHandler[SimpleSpan]):
                     # start new tree
                     tree = Tree()
 
-            tree.create_node(
-                tag=f"{span.id_} ({span.duration})",
-                identifier=span.id_,
-                parent=span.parent_id,
-                data=span.start_time,
-            )
+            try:
+                tree.create_node(
+                    tag=f"{span.id_} ({span.duration})",
+                    identifier=span.id_,
+                    parent=span.parent_id,
+                    data=span.start_time,
+                )
+            except NodeIDAbsentError:
+                warnings.warn("Parent with id {span.parent_id} missing from spans")
+                # create new tree and fake parent node
+                trees.append(tree)
+                tree = Tree()
+                tree.create_node(
+                    tag=f"{span.parent_id} (MISSING)",
+                    identifier=span.parent_id,
+                    parent=None,
+                    data=span.start_time,
+                )
+                tree.create_node(
+                    tag=f"{span.id_} ({span.duration})",
+                    identifier=span.id_,
+                    parent=span.parent_id,
+                    data=span.start_time,
+                )
+
         trees.append(tree)
         return trees
 
diff --git a/llama-index-core/llama_index/core/response_synthesizers/base.py b/llama-index-core/llama_index/core/response_synthesizers/base.py
index ebedd93aa6f906b7d8e5a2fef8870c437728cb55..b8440807898a5e1486cdba4be74212f09c5c7f3c 100644
--- a/llama-index-core/llama_index/core/response_synthesizers/base.py
+++ b/llama-index-core/llama_index/core/response_synthesizers/base.py
@@ -209,13 +209,13 @@ class BaseSynthesizer(ChainableMixin, PromptMixin):
                     response_gen=empty_response_generator()
                 )
                 dispatcher.event(
-                    SynthesizeEndEvent(query=query, response=empty_response)
+                    SynthesizeEndEvent(query=query, response=str(empty_response))
                 )
                 return empty_response
             else:
                 empty_response = Response("Empty Response")
                 dispatcher.event(
-                    SynthesizeEndEvent(query=query, response=empty_response)
+                    SynthesizeEndEvent(query=query, response=str(empty_response))
                 )
                 return empty_response
 
@@ -240,7 +240,7 @@ class BaseSynthesizer(ChainableMixin, PromptMixin):
 
             event.on_end(payload={EventPayload.RESPONSE: response})
 
-        dispatcher.event(SynthesizeEndEvent(query=query, response=response))
+        dispatcher.event(SynthesizeEndEvent(query=query, response=str(response)))
         return response
 
     @dispatcher.span
@@ -258,13 +258,13 @@ class BaseSynthesizer(ChainableMixin, PromptMixin):
                     response_gen=empty_response_agenerator()
                 )
                 dispatcher.event(
-                    SynthesizeEndEvent(query=query, response=empty_response)
+                    SynthesizeEndEvent(query=query, response=str(empty_response))
                 )
                 return empty_response
             else:
                 empty_response = Response("Empty Response")
                 dispatcher.event(
-                    SynthesizeEndEvent(query=query, response=empty_response)
+                    SynthesizeEndEvent(query=query, response=str(empty_response))
                 )
                 return empty_response
 
@@ -289,7 +289,7 @@ class BaseSynthesizer(ChainableMixin, PromptMixin):
 
             event.on_end(payload={EventPayload.RESPONSE: response})
 
-        dispatcher.event(SynthesizeEndEvent(query=query, response=response))
+        dispatcher.event(SynthesizeEndEvent(query=query, response=str(response)))
         return response
 
     def _as_query_component(self, **kwargs: Any) -> QueryComponent: